This invention relates to disks used to grind material into particulate of predetermined size and, more particularly, to grind coffee beans into ground coffee of a predetermined size.
Grinding disks or burrs have been in use for centuries to process food stuffs such as grain into flour and the like. Typically, one burr is held stationary with respect to the other, the rotating burr. The material to be ground is metered between the opposing surfaces thereof usual through openings or apertures in the center of the burrs. The material is then moved across the surfaces of the burrs under pressure and diminished into fines. To promote the grinding action, the burrs may be made abrasive through the use of tough, abrasive material coating the face of the burr or, as is more typical today, the use of pins or teeth integral with the burr disks. In coffee bean grinders, the teeth of the two disks "nest" together and cause the material to be broken and the ground into the final size. ("Nest", "nested", or "nesting" for purposes of this description means the teeth on one disk have a juxtaposed but slightly spaced relationship to the teeth on the other disk.) An early example of a nested pair of grinding disks or burrs is illustrated in U.S. Pat. No. 492,000 issued May 9, 1911 to Kiregreen. The nesting of the teeth permitted the beans to be ground therebetween.
Apparatus using grinding burrs having teeth or serrated surfaces for grinding material are legion. U.S. Pat. Nos. 4,967,649 and 5,052,814 both issued to Ephraim et al on Nov. 6, 1990 and Oct. 22, 1991, respectively, provide discussions on how the coffee beans are metered into the grinding burrs. The techniques of metering beans into the grinder does not form a part of the present invention and a discussion is not deemed necessary for an understanding of the present invention. To the extent such a discussion is considered important, then the substance of such patents are incorporated by way of reference herein for such understanding.
To more clearly discuss such burrs and the various dimensions making the burr, reference is made to FIGS. 1-3. FIG. 1 represents in schematic form a typical grinding burr tooth 10. The leading face 12 of tooth 10 is shown in the direction of rotation 14 of disk 16. The trailing face 18, as its name implies, trails the leading face 12 during rotation. Each face 12 and 18 has a triangular shape sloping inwardly, thus forming a slope angle with the vertical. Connecting the faces 12 and 18 are a pair of lateral faces 20 and 22 converging toward each other in an upward direction from disk 16. The line of intersection 24 of faces 20 and 22 extends between the apices of faces 12 and 18.
FIG. 2 depicts a portion of disk 16 having a grouping of teeth 10 positioned in circular rows having center of curvatures coinciding with the center of disk 16. Adjacent rows of teeth 10 form "tangential valleys" 26 that circle the center of disk 16. As seen in FIG. 2, adjacent teeth 10 form V-shaped valleys 28 called radial valleys. The line of intersection of each leading face 12 with the surface of disk 16 lies along the bottom of each radial valley 28. The pattern of teeth of a typical coffee bean grinding burr disk is formed so that the line of intersection in an inner row is coextensive with a line of intersection in the next row. Dashed line 30 represents this coextensive structure. Thus, assuming there is no obstruction, a particulate of the material being ground would "see" a clear path or exit from the first row of teeth to the perimeter of the disk through a connected series of radial valleys. Despite the descriptive adjective "radial", radial valleys are generally not coextensive with radial lines or rays drawn from the center of the burr disk. For reasons to be described below, it is desirable that the connected radial valleys (and therefore extension line 30) be at an angle or skewed with respect to a ray of the disk intersecting the extension line at start of the line of intersection of the leading face of a tooth in the first or innermost row. The skewing of the radial valleys 28 to a ray 32 is illustrated in FIG. 3. Circle 34 represents the innermost row of teeth 10. Dashed line 36 represents a connected group of radial valleys 28 and forms an angle 38, the skew angle, with ray 32. The skew angle may be more specifically defined as the angle between the coextensive line formed by the intersections of the leading faces with a ray since, as will be discussed below, some of the teeth in the innermost rows are spaced from each other and thus do not form the V-shaped valleys.
The teeth of a grinding burr are generally grouped and characterized by results desired from the teeth. For example, the teeth of the inner most row of teeth are the largest teeth on the burr disk and are called cracking teeth. Cracking teeth are spaced far enough apart to allow the coffee beans to move therebetween. Thus, the distance between teeth in the first row must exceed the average diameter of the coffee beans to be ground. This distance is preferably about 3 centimeters or greater. The coffee beans metered between the two disks are first cracked by the nested cracking teeth, with the resulting particles being extruded between the inside lateral face of the stationary teeth and the outside lateral face of the rotary teeth. The particles are scrapped off by the cutting edge of the next adjacent tooth, the cutting edge being defined as the edge between the leading face and the inside lateral face. The extruding action continues until the particles reach an exit, a radial valley that it is not blocked by a nested tooth. Then by a combination of forces, centrifugal and feed forces, the particles proceed outwardly from the center of the disk through the open radial valley into the next tangential valley. The extruding action continues until the particles reach the next exit. As the particles move steadily outward toward the perimeter of the disk, the size of the teeth become smaller as does the average gap between the nested teeth. The outermost teeth are frequently referred to as sizing teeth while the those in the middle are often called intermediate teeth.
A careful examination of the burrs currently used in the grinding of coffee has resulted in a determination by applicant that the gaps or clearances between nested teeth are uneven. The inside lateral face of a tooth is often spaced a different distance from its associated nested tooth than the outside lateral face. Applicant then noted that the uneven clearances or gaps frequently caused flow pinch points that resulted in diminished through put during the grinding process and also resulted in increased power consumption.
Upon further analysis of current prior art coffee bean grinding burrs, it was noted that the "slope angle" of the leading face of the teeth of the burrs were about 45.degree. while the trailing faces had slope angles of between 30.degree. and 45.degree.. Slope angle is defined as the angle between the sloping face and a line vertical to the surface of the disk. Applicant has determined that the proper selection of the slope angles of both the leading and trailing faces is an important factor in the improved performance of the grinding burrs.
Still another factor determined by applicant to be important, yet apparently not considered by those skilled workers in the prior art, is the construction of burrs with the proper skew angles. Applicant has noted that the "combined skew angles" of the current prior art grinding burrs of coffee bean grinders do not exceed about 92.degree.. For purposes of this description, combined skew angle is defined as the sum of the skew angle of the rotary burr disk and the stationary burr disk. Applicant has found that, by the careful selecting the combined skew angle, the performance of the coffee bean grinders using the grinding burrs can be further improved.
Finally, applicant determined that the prior art coffee bean grinding burr designs generally ignore the relative depth of the radial valleys and tangential valleys, particularly with respect to the final sizing teeth. For example, when burrs are adjusted to provide coffee fines such as used with espresso where the mean particle size is about 0.010 inches, it has been observed that the radial valleys of prior art burrs are often higher than the tangential valleys, resulting in a jamming of the coffee paths. This causes an undesired heating of the fines and larger power consumption.
These findings as the result of applicant's analysis of prior art coffee bean grinders has led applicant to address the objectives set forth below.
It is a paramount object of the present invention to provide for an improved coffee bean grinding burr having superior grinding rates.
It is another object of the present invention to provide for coffee bean grinding burrs that minimize flow pinch point.
It is still another object of the present invention to provide for improved coffee bean grinding burrs minimizing jamming of coffee bean fines.
These and other objectives will become apparent to those skilled in the art after a reading the following description and appended drawing.